1. Field of the Invention
The present invention relates to an indication display unit for vehicles which provides a driver with the whole information concerning its navigation such as its velocity, revolution frequency of the engine thereof and so on, and more particularly to an indication display unit, in which an indicated pattern (or image) projected from an indication projecting device is reflected on the windshield so that the driver can catch sight of its virtual image further therebehind.
2. Description of the Prior Art
An indication display unit called a "head-up display" as shown in FIG. 5 is widely known as one of the conventional display units.
In the above FIG. 5, the head-up display unit 2 disposed near the windshield 1 is provided with a lens 4 just behind the dashboard 3, and a display device 5 such as an LCD (Liquid Crystal Display), and a light source 6 at the opposite surface thereto.
As shown in the same figure, the pattern 7 indicated on the display device 5 is projected by the light source 6 disposed underneath thereof so as to be finally reflected on the windshield 1 through the lens 4, resulting in that the driver can catch sight of the reflected pattern on the windshield 1 as though it is overlapped with the outside scenery behind thereof.
In this type of the display unit, however, when the sunbeams are directly radiated to the lens 4 of the head-up display unit along the optical axis, the radiated sunbeams are collected on the focal point 10 through the lens 4 so that the display device 5 disposed near there is directly radiated. In this case above, the heating temperature rises quite high in a substantially short time so as to cause the display device 5 to be easily destroyed, especially in the summer time.
In order to solve the above problems, a device for detecting the sunbeams radiated into the lens unit has been disclosed, whereby the radiation of the sunbeams is prevented by use of a shutter plate disposed between the lens and the display device.
FIG. 7 shows the above type of device, in which a semi-translucent mirror 11 is disposed between the lens 4 and the display device 5, so that a part of the incident sunbeams entering through the lens is reflected on the semi-translucent mirror 11 so as to be conducted onto the light detecting element 13.
The signal sent from the light detecting element 13 is inputted to the CPU 19 through an A/D converter, and when the CPU 19 judges that the above light detecting element 13 has detected the light exceeding the predetermined level, a shutter driving circuit 20 is activated by a drive control circuit 21 so as to energize a shutter plate 12.
In the same figure, 14 denotes a protecting case, 15 a blower driver, 16 a display device activating circuit, 17 a temperature detecting element, 18 an A/D converter, 22 an A/D converter, 23 a blower activating circuit, 24 a light source activating circuit, 26 a ventilator and 27 a blower.
However, since the above device has adopted the semi-translucent mirror for conducting the light incident from the external world to the light detecting element in the same path for projecting the above indicated pattern, the light for projecting the above pattern generated in the LCD cell is reduced while it passes through the semi-translucent mirror, whereby the brightness of the indicated pattern which is finally reflected on the projecting surface, such as a windshield, is greatly reduced.